The invention relates to the technical field of producing medium and low pressure plasmas (of the order of a few hundredths of a Pascal to a few tens of Pascals) by capacitive-type discharges, the plasma being of uniform character suitable for being controlled firstly over large areas whether plane or otherwise, and secondly over a wide range of ion densities from high (up to 1012 cmxe2x88x923) to very low (108 cmxe2x88x923).
The invention finds particularly advantageous applications in all fields that require uniform plasmas at medium or low pressure to be produced over large areas, for performing surface treatments (cleaning, etching, deposition) and, in particular, for plasma deposition of polymers or for providing independent control of the energy with which a surface or a substrate is subjected to ion bombardment.
In conventional manner, apparatus for producing a plasma by capacitive-type discharges comprises a passive electrode placed at a distance from an active electrode. The electrodes are mounted inside an enclosure containing gas at a substantially constant pressure. The passive electrode is placed at an electric potential, e.g. a reference potential such as ground, while the active electrode is fed with a voltage for maintaining discharge. The electric voltage applied to the active electrode is either a DC voltage which is negative relative to the passive electrode, or else a low frequency AC voltage, or more commonly a radiofrequency (RF) voltage (13.56 MHz or a multiple or a submultiple of 13.56 MHz) applied through a capacitance of low impedance. Above some value of applied voltage (which depends on the nature of the gas, on its pressure, on the distance between the electrodes, on the frequency of the applied signal, and on the nature of the electrodes and their relative dimensions), a discharge is struck between the two electrodes.
Whatever the frequency at which the voltage is applied, one of the electrodes is always negatively polarized relative to the other. Since the potential of the plasma generally takes up a value which is slightly positive relative to the more positive electrode, the more negative electrode is subjected to intense bombardment by the positive ions of the plasma that are accelerated from the plasma towards the more negative electrode through the ion sheath separating the plasma from the electrode. During this time, the more negative electrode is subjected to ion bombardment at very low energy only, insofar as its potential is then very close to the potential of the plasma.
When the ions strike the electrode that is more negative relative to the potential of the plasma, they generally emit secondary electrons which are those that are accelerated in the opposite direction towards the plasma. On passing through the inter-electrode gap, these so-called xe2x80x9cfastxe2x80x9d secondary electrons produce the plasma, i.e. they produce slow or thermal electrons and ions. If the mean free path of the fast electrons is greater than the inter-electrode distance (as applies at low and medium pressures), most of the fast electrons emitted by the negative electrode are subjected to few inelastic collisions, so they are not slowed down and they are therefore collected by the positive electrode. In contrast, if the mean free path of the fast electrons is shorter than the inter-electrode distance (as applies at higher pressures), then the fast electrons are slowed down by the inelastic collision processes and can in turn become slow electrons. Under steady conditions, the ions and the slow electrons produced within the plasma are lost to the walls in equal quantities, which is a condition necessary for maintaining macroscopic neutrality of the plasma.
Apart from the fast electrons emitted by the bombardment of the negative electrode of the discharge, some electrons can also be accelerated most effectively by the periodic electric field within the ion sheath. When the voltage of the electrode becomes more negative relative to the potential of the plasma, the ion sheath becomes larger and the electrons present are then pushed away and accelerated towards the plasma by the electric field which develops within the sheath. Naturally, the ions present in the sheath are accelerated in the opposite direction by the electrode. This dynamic behavior of the sheath can contribute significantly to producing fast electrons and thus to producing plasma.
A drawback of a capacitive discharge is that for a given geometrical configuration of the discharge, the conditions required for striking and above all for maintaining the discharge require a threshold voltage to be applied between the electrodes, i.e. require a threshold power in order to maintain the discharge. This puts a limit on the possibilities of producing plasma at low densities. In other words, whereas an increase in plasma density can be obtained merely by increasing the power injected into the discharge, it is not possible to decrease its density towards the lowest values.
As explained above, the operation of a capacitive discharge is determined by:
the configuration of the discharge (inter-electrode distance and ratio of electrode areas);
the nature of the gas (effective collision section);
the pressure of the gas (collision frequencies and mean free paths);
the nature of the electrodes (secondary electron emission rates);
the frequency of the applied voltage (sheath dynamic behavior, ion bombardment energy); and
the electric power injected into the plasma.
The above parameters define the conditions of the discharge completely, i.e. they define its operating point. Simultaneously, those parameters govern the plasma-surface interaction parameters which are totally dependent on the operating point of the capacitive discharge. It is therefore not possible for plasma production to be independent from plasma interaction with the surface to be treated that is placed on one of the electrodes. In other words, it is not possible to adjust ion bombardment energy without modifying the power delivered to the discharge and thus the electrical characteristics of the discharge (density, electron temperature, etc.).
One option in general use for remedying that drawback is to add one or more bias electrodes for the purpose of modifying the current balance over the electrodes. Nevertheless, the additional degrees of freedom for the plasma parameters that are obtained by that method are largely counteracted by the complexity of the situation created in that way.
An object of the invention is thus to remedy the above drawback by proposing a method of producing a plasma by capacitive-type discharges produced between an active electrode and a passive electrode, the passive electrode being placed at a given electric potential, e.g. a reference potential, while the active electrode is fed with a discharge-maintaining voltage.
To achieve such an object, the method of the invention consists in placing a multipole magnetic barrier between the electrodes, the magnetic field lines thereof extending across a separation plane parallel to the electrodes so as to ensure that the fast electrons accelerated by the active electrode are caused to oscillate between the poles in order to create plasma production and diffusion zones that are situated on either side of the magnetic barrier facing each of the electrodes.
Another drawback inherent to capacitive discharges is the existence of great non-uniformity in the density of the plasma due in particular to edge effects which can be very large. The only known means used for remedying that defect is to adjust or distribute gas injection in such a manner as to correct the non-uniformity of the method implemented.
Another object of the invention is to propose a method of producing a plasma by capacitive-type discharges, the method being adapted to obtaining capacitive discharge strikes at very low or very high levels of power per unit area, while nevertheless obtaining a plasma that is uniform over large surface areas.
To achieve this object, the method of the invention consists:
in making the active electrode out of a series of individual active electrodes facing the passive electrode and each defining both an active surface and a passive surface;
in distributing the individual active electrodes in a geometrically uniform manner facing the passive electrode in order to obtain a uniform plasma; and
in feeding and active surfaces of the individual active electrodes with a respective discharge-maintaining voltage.